The success of marrow transplantation is heavily dependent on the transfusion of blood products and is a situation which provides many opportunities for research in transfusion immunology. In Project IX we have described our research proposals for problems of platelet alloimmunization. In Project III we describe our proposals for the utilization of blood products screened for antibody to cytomegalovirus (CMV) as well as an evaluation of leukocyte-depleted blood products for the prevention of CMV. In this core section we are concerned with mechanisms for deriving research information from clinically necessary transfusion support and plasma modification procedures. We ask support only for key personnel for supervision and data entry where we cannot recover costs from charges but where the work is essential to carrying out the proposed research in individual program projects. These areas include establishing and maintaining data bases containing detailed descriptive information on all blood product transfusions and on the course and outcome of all ABO-incompatible transplants requiring plasma modification or marrow red cell depletion.